Tracking error detecting apparatus for use in multibeam optical disk device

ABSTRACT

In a multi-beam optical disk device in which a signal is recorded on or reproduced from an optical disk by means of a plurality of light beams, a tracking error detecting apparatus is constructed such that the plurality of light beams projected from an optical head are made incident upon the optical disk and one of these light beams is made incident upon a groove formed in the optical disk and a first tracking error signal is derived from the one light beam reflected by or transmitted through the groove, and all the light beams are made incident upon a groove formed in the optical disk and a second tracking error signal is derived from all the beams reflected by or transmitted through the groove.

BACKGROUND OF THE INVENTION

The present invention relates to a tracking error detecting apparatusfor use in a multi-beam optical disk device in which signals arerecorded or reproduced by using a plurality of beam spots.

In an optical disk device, a record track on an optical disk is tracedwith the aid of a light beam spot by moving an optical head in a radialdirection of the disk while the optical disk is rotated. In order totrace the record track correctly by the light beam, the tracking controland focusing control are effected.

The tracking control has been carried out by various methods, and atypical push-pull method (sometimes called far field method) will beexplained. As shown in FIG. 1a, FIG. 1b and FIG. 1c, in a surface of anoptical disk 1 there are formed grooves 2, and a light beam spot isprojected on the disk surface by means of an objective lens 3. Adistribution of the intensity of light reflected by the disk 1 is shownby P. FIG. 2 illustrates the construction of a tracking error detectingcircuit in which output signals from two-divided photodiode 4 having twoelements 4a and 4b are supplied to a differential amplifier 5 which thenproduces a tracking error signal T.

Upon the two elements 4a and 4b of the two-divided photodiode 4 is madeincident a light spot S of the light beam reflected from the disk. FIG.1b shows the case in which the tracking is correctly attained, andintensity of light spot S formed on each of the elements 4a and 4b isequal to each other. Therefore, the tracking error signal T generated bythe differential amplifier 5 is zero.

FIG. 1a and FIG. 1c depict the cases in which the tracking is notcorrectly attained, and the center of the light beam spot is deviatedfrom the center of the groove 2. In this case the intensities of thelight spot S reflected from the optical disk 1 and impinging upon theelements 4a and 4b are not identical with each other. Therefore, thedifferential amplifier 5 generates the tracking error signal T havingpositive or negative polarity and the tracking error signal T thusobtained is used to perform the tracking control.

The focusing control has been effected in various methods. In FIG. 3,the light reflected by the optical disk 1 is made incident upon a photodetector 8 by means of the objective lens 3 and cylindrical lens 7. Thephoto detector 8 is divided into four light receiving elements 8a, 8b,8c and 8d as illustrated in FIG. 4. Output signals Ia, Ib, Ic and Idgenerated from the elements 8a, 8b, 8c and 8d, respectively are added byadders and then are supplied to a differential amplifier 9 to derive afocusing error signal F=(Ia+Ib)-(Ic+Id).

When the optical disk 1 is in the in-focus position, the light beamreflected by the optical disk 1 forms a circular light beam spot S onthe light receiving element 8 as shown in FIG. 5b, so that the focusingerror signal F becomes zero. However, when the optical disk 1 is in theout-of-focus position, the reflected light beam spot S having a shapeshown in FIG. 5a or FIG. 5c is formed on the light receiving element 8,and thus the focusing error signal F having positive or negativepolarity is generated. The objective lens 3 is moved by the automaticfocusing control mechanism to attain the correct focus condition.

FIG. 6 is a view showing a whole construction of the above mentionedtracking control and focusing control. In the focusing control system,the focusing error signal F generated by the differential amplifier 9 isamplified by an amplifier 10a and then is supplied to a lens drivingmechanism 11f which moves the objective lens 3. In the tracking controlsystem, the tracking error signal produced by the differential amplifier5 is supplied via an amplifier 10b to a lens driving mechanism 11t tomove the objective lens 3. J1 denotes a slider signal for accessing adesired track, and when the optical head is moved in the radialdirection to access a desired track, the slider signal is supplied via adriving amplifier 10c to a slider 12. To this end the optical head isplaced on rollers 13. The driving amplifiers 10a and 10b have afrequency range of several tens k Hz to effect the fine trackingcontrol, while the driving amplifier 10c has a frequency range ofseveral k Hz to effect the coarse tracking control. A switch 14 isprovided to select either the slider signal for retrieval J1 or thetracking signal T, and a switch 15 is provided for selecting one of thetracking signal T and a track jump signal J2.

In order to access a given track on the optical disk by moving theoptical head over a long distance, the switch 14 is connected to J1 sideand the tracking control system is disconnected. By means of the slidersignal J1, the slider 12 is accelerated into a given direction for agiven time period and then is decelerated. Next the switch 15 isswitched into the side of the track jump signal J2 and the light spot isjumped over one track by supplying a pulse current. An address signalrecorded in a track is checked to derive the number of tracks over whichthe light spot has to be jumped until a desired track is accessed, andthe track jump is repeated by the detected number of times. After thedesired track has been accessed, the switches 14 and 15 are driven intothe side of the differential amplifier 5, and then the usual trackingcontrol is carried out with the aid of the tracking error signal T.

In a multi-beam optical disk device in which a plurality of light beamsare projected from a single optical head and a plurality of tracks aresimultaneously recorded or reproduced in order to increase the datatransfer rate, it is also possible to derive the focusing error signal Fand tracking error signal T by similar methods to those explained above.

In order to effect the track jump accurately each time the track accessis to be performed in the multi-beam optical disk device, it isnecessary to derive tracking error signals for respective light beamspots. For instance, a tracking error detecting circuit of the push-pullmethod shown in FIG. 7a and FIG. 7b may be used. In FIG. 7a, thetwo-divided photodiode 4 illustrated in FIG. 2 is used for the fourlight beams, and in FIG. 7b, a plurality of two-divided photodiodes 4are used. The function of the device shown in FIG. 7b is substantiallysame as that of the device depicted in FIG. 7a. A reference numeral 16denotes an adder.

In the push-pull method, a difference in intensity of first orderinterference light in the perpendicular direction to the track directiondue to the groove 2 of the optical disk 1 is detected, so that a signalsimilar to the tracking error signal is derived even when the objectivelens 3 is shifted in the tracking direction with respect to the opticalaxis and when the optical disk is inclined. Therefore, the zero point ofthe tracking error signal is shifted to produce the tracking off-set.That is a fault of the push-pull method. Although the tracking errorsignal is obtained from a predetermined single light beam, the trackingoff-set signal is increased by n times as compared with the trackingoff-set which is obtained by a signal light beam, wherein n is thenumber of light beams which are used simultaneously. In practice, thereis obtained a signal which is a mixture of the tracking error signal andthe tracking off-set signal and it is difficult to extract only thetracking error signal.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a tracking errordetecting apparatus for use in the multi-beam optical disk device inwhich the recording or reproducing is carried out for a plurality ofrecord tracks with the aid of a plurality of light beams projected by anoptical head.

According to the invention a predetermined light beam among a pluralityof light beams is selectively made incident upon a groove and thetracking error signal is derived by processing said predetermined lightbeam reflected by or transmitted through the optical disk while thegeneration of the tracking off-set can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a, 1b, 1c and FIG. 2 are explanatory views for explaining thetracking control of the push-pull method;

FIGS. 3, 4, 5a, 5b and 5c are explanatory views for explaining thefocusing control using a four-divided photodiode;

FIG. 6 is a view showing a whole construction of tracking control andfocusing control.

FIG. 7a and FIG. 7b show the electrical construction of a tracking errordetecting circuit of multi-beam push-pull method; and

FIGS. 8 to 13b are views for explaining embodiments of the trackingerror detecting apparatus for using the multi-beam optical disk deviceaccording to the invention, in which:

FIG. 8 is a perspective cross sectional view showing the optical disk;

FIG. 9a shows the construction of the optical system of a firstembodiment;

FIG. 9b is a circuit diagram of the first embodiment;

FIG. 9c is an explanatory view representing the function of the pinhole;

FIG. 9d and FIG. 9e are waveforms of the tracking error signal;

FIG. 10b depicts the construction of the optical system of a secondembodiment;

FIG. 10a is an explanatory view for explaining the function of thecritical angle prism;

FIGS. 11a and FIG. 11c show the construction of the optical system of athird embodiment;

FIG. 11b is a circuit diagram of the third embodiment;

FIG. 12a shows the optical system of a fourth embodiment;

FIG. 12b is a circuit diagram of a fourth embodiment;

FIG. 13a is a schematic view showing the optical construction of a fifthembodiment; and

FIG. 13b is a circuit diagram illustrating the electrical constructionof the fifth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained in detail with reference toembodiments illustrated in figures succeeding FIG. 8.

FIG. 8 is an explanatory figure for explaining the embodiments commonly.An optical disk 21 comprises transparent substrate 21a, record medium21b and protection layer 21c. In the transparent substrate 21a there areformed grooves 21d, and on the substrate are applied the record medium21b and protection layer 21c. A portion of the record medium except forthe grooves 21d is called the land 21e, and a reference numeral 21fdenotes center lines of record tracks. Further reference characters A,B, C and D represent light beam spots which are projected from the lowersurface of the optical disk 21. One light beam spot B selected among thefour light beam spots A to D is made incident upon the groove 21d. Thatis to say, among the four light beam spots A to D the light beam spot Bwhich is closest to the optical axis of the optical system is madeincident upon the groove 21d. Since the focusing error detection isliable to be affected by the distortion of the optical system, the lightbeam spot B closest tot he optical axis is used for the deviation, saidlight beam spot B being affected by the distortion to the least extent.

FIG. 9a shows a first embodiment in which the light beam B for detectingthe focusing and tracking errors is separated from the remaining lightbeam spots A, C and D with the aid of a pin hole. FIG. 9a shows theoptical system in which between the optical disk 21 and a laser array 22for producing four light beams are arranged collimator lens 23, halfmirror 24 and objective lens 25 in this order. In an optical path of alight beam reflected by the half mirror 24 are arranged half mirror 26,condenser lens 27, pin hole plate 28 condenser lenses 29, 30 cylindricallens 31 and four-divided photodiode 32. In an optical path of a lightbeam reflected by the half mirror 26 are arranged condenser lens 33 andtwo-divided photodiode 34.

FIG. 9b depicts the electrical construction, and the four-dividedphotodiode 32 is divided into four elements 32a, 32b, 32c and 32d.Output of these elements are connected to adders 35a, 35b, 35c and 35d,and outputs of these adders are connected to differential amplifiers 36aand 36b. The differential amplifier 36a produces the focusing errorsignal F and the differential amplifier 36b produces the tracking errorsignal T1 which is supplied to a switch 37. The two-divided photodiode34 is divided into two elements 34a and 34b whose outputs are connectedto a differential amplifier 36c and an output of this differentialamplifier is connected to the switch 37. Then the switch 37 serves toselect one of the output signals of the differential amplifiers 36b and36c.

The four light beam spots A, B, C and D reflected by the optical disk 21are reflected by the half mirror 24, transmitted through the half mirror26 and then are converged by the condenser lens 27. Since there isprovided the pin hole plate 28, the beam spots A, C and D are shieldedthereby, and only the beam spot B is transmitted through the pin holeplate 28 as shown in FIG. 9c. The beam spot B is then made incident uponthe four-divided photodiode 32 by means of the condenser lenses 29, 30and cylindrical lens 31. The four-divided photodiode 32 has the similarfunction to that explained with reference to FIG. 3 to FIG. 5, and thefocusing error signal F can be obtained by deriving a difference betweenthe output signals of the adders 35a and 35b by the differentialamplifier 36a. Further the tracking error signal T1 similar to thatexplained above with reference to FIG. 1 and FIG. 8 can be obtained byderiving a difference by the differential amplifier 36b between theoutput signals of the adders 35c and 35d.

A part of the light reflected by the optical disk 21 is reflected by thehalf mirrors 24, 26 and is made incident upon the two-divided photodiode34 by means of the condenser lens 33. As shown in FIG. 7a, thetwo-divided photodiode 34 operates as the tracking error detector of thepush-pull method for the multi-beam optical disk device. As explainedabove with reference to FIG. 7, the zero point of the tracking errorsignal T2 produced by the differential amplifier 36c is shifted due tothe inclination of the optical disk 21, so that it does not representthe true tracking error. In order to effect the track jump and trackaccess precisely, it is necessary to obtain the tracking error signalaccurately for respective tracks, because it is practically difficult toeffect the track jump for a set of four tracks. To this end the beamspots A, B, C and D are projected by turns on the groove 21d to derivethe tracking error signal by the two-divided photodiode 34. At first,the switch 37 is connected to the side of the differential amplifier 36cso that the beam spot B can trace a given track. Then the switch 37 ischanged into the side of the differential amplifier 36b and therecording, reproducing or erasing operation is carried out.

FIG. 9d depicts the tracking error signal T2 which is obtained when theswitch 37 is connected to the side of the differential amplifier 36c toeffect the track jump or track access. Since the beam spots A, B, C andD are obtained by the light beams which are projected on the groove 21dsuccessively, there are produced the tracking error signals forrespective beam spots A to D.

FIG. 9e represents the tracking error signal T1 which is produced whenthe switch 37 is connected to the side of the differential amplifier 36bto effect the recording, reproducing or erasing. The tracking errorsignal is obtained only by the beam spot B of the light beam which isprojected on the groove 21d.

FIGS. 10a and 10b illustrate a second embodiment in which only the beamspot B is used for detecting the focusing error and tracking error, andthe remaining beam spots A, C and D are removed by means of a criticalangle prism 38. FIG. 10a shows the optical construction using twocritical angle prisms 38a and 38b instead of the condenser lens 27, pinhole plate 28 and condenser lens 29 shown in FIG. 9a. FIG. 10b is anexplanatory view for explaining the function of the critical angle prism38. It should be noted that the electrical construction of the secondembodiment is the same as the first embodiment shown in FIG. 9b. Due tothe function of the critical angle prisms, the critical angle prisms 38aand 39b extract only the light beam spot B among the light beam spots Ato D and the extracted beam spot B is made incident upon thefour-divided photodiode 32 by means of the condenser lens 30 andcylindrical lens 31. Then there are obtained the focusing error signal Fand two tracking error signals T1 and T2 in the similar manner to thatof the embodiment shown in FIG. 9b.

In the second embodiment, the tracking error detecting beam spot and thefocusing error detecting beam spot are commonly formed by the same beamspot, and the photodiode for detecting the tracking error signal and thephotodiode for detecting the focusing error signal are also commonlyformed by the same photodiode.

FIGS. 11a, 11b and 11c show a third embodiment in which only the beamspot B among the four beam spots A to D is used to detect the focusingerror and tracking error and the four-divided photodiode 32 is formed tobe sufficiently small for being illuminated only by the beam spot B. Asillustrated in FIG. 11a the condenser lens 27, pin hole plate 28 andcondenser lens 29 shown in FIG. 9a are all removed, and the size of theelements 32a to 32d of the four-divided photodiode 32 is small withrespect to the beam spots A to D as depicted in FIG. 11b. It should benoted that the construction shown in FIG. 11a may be altered in a mannerillustrated in FIG. 11c.

In the third embodiment explained above, the same beam spot B is used asboth the tracking error detecting beam spot and focusing error detectingbeam spot, and the tracking error detecting photodiode and focusingerror detecting photodiode are commonly formed by the same photodiode.

FIGS. 12a and 12b show a fourth embodiment in which only the beam spot Camong the four beam spots A to D is used as the focusing error detectionand the beam spot C is divided into two beam spots at a point before thefour-divided photodiode 32, and one of which is made incident upon afirst two-divided photodiode 34 and the other of which is made incidentupon a second two-divided photodiode 41. As illustrated in FIG. 12a,between the half mirror 26 and the condenser lens 30 is arranged a halfmirror 39 and in an optical path of a light beam reflected by the halfmirror 39 are arranged condenser lens 40 and two-divided photodiode 41.

FIG. 12b depicts the electrical construction in which output signalsfrom the four-divided photodiode 32 are processed by the adders 35a, 35band differential amplifier 36a to derive the focusing error signal Fsimilar to that shown in FIG. 4. Output signals from elements 41a and41b of the two-divided photodiode are supplied to a differentialamplifier 36d which produces the tracking error signal T1 similar tothat explained in connection with FIG. 8. One of the tracking errorsignal T1 and tracking error signal T2 produced by the differentialamplifier 36c is selected by the switch 37.

In the first, second and third embodiments shown in FIGS. 9, 10 and 11,respectively, both the tracking error and focusing error are detected bycommonly using the same beam spot and the same photodiode, so that theprecision of the error detection is somewhat decreased due to the mutualinterference. However, in the fourth embodiment illustrated in FIG. 12the tracking error is detected by using the beam spot B and two-dividedphotodiode 41 and the focusing error is detected by using the beam spotC and four-divided photodiode 32, so that the tracking control andfocusing control can be performed accurately without being affected bythe mutual interference.

Similar to the first embodiment, upon effecting the track jump or trackaccess, the switch 37 is first connected to the side of the differentialamplifier 36c and after a given track has been accessed the switch 37 ischanged into the side of the differential amplifier 36d to effect therecording, reproducing or erasing.

FIGS. 13a and 13b depict a fifth embodiment in which among the four beamspots A to D the beam spot B is selectively used for detecting thetracking error and the beam spot C is used to detect the focusing error,and the beam spot B is projected on the groove in the optical disk 21.The optical construction shown in FIG. 13a is substantially same as thatillustrated in FIG. 11a, but the light flux transmitted through the halfmirror 26, condenser lens 30 and cylindrical lens 31 is made incidentupon the four-divided photodiode 32 and two-divided photodiode 42. Itshould be noted that these photodiodes 32 and 42 are integrally formedwith each other and are arranged in the direction of the array of thefour beam spots A to D as shown in FIG. 13b. The four-divided photodiode32 and two-divided photodiode 42 are formed by relatively smallphotodiodes so that only the light beam spots B and C are made incidentupon these photodiodes 32 and 42, respectively.

As explained with reference to the previous embodiments, output signalsof the four-divided photodiode 32 are processed by the adders 35a, 35band differential amplifier 36a to derive the focusing error signal F.Output signals from the elements 42a and 42b of the two-dividedphotodiode 42 are supplied to the differential amplifier 36e to producethe tracking error signal T1, and output signals from the two-dividedphotodiode 34 are supplied to the differential amplifier 36c to derivethe tracking error signal T2 similar to that explained above inconnection with FIG. 9. One of these tracking error signals T1 and T2 isselected by the switch 37.

In the fifth embodiment illustrated in FIG. 13, the four-dividedphotodiode 32 for detecting the focusing error and the two-dividedphotodiode 42 for detecting the tracking error are integrally formed,but they are electrically separated from each other, so that the mutualinterference can be removed. Therefore, the tracking control andfocusing control can be performed precisely.

Similar to the first embodiment shown in FIG. 9, when the track jump ortrack access is to be effected, the switch 37 is first connected to theside of the differential amplifier 36c and after a given track has beenaccessed, the switch 37 is changed into the side of the differentialamplifier 36e to perform the recording, reproducing or erasing.

As explained above in the first to third embodiments the single beamspot is extracted from a plurality of beam spots by means of the pinhole plate and crystal angle prisms or by using the four-dividedphotodiode having a small area and the tracking error and focusing errorcan be detected without being affected by the remaining beam spots.

In the first to third embodiments, among a plurality of beams the singlebeam situated on the optical axis or nearest to the optical axis of theoptical system is projected upon the groove in the optical disk 21, andthe light beam transmitted through or reflected by the groove isprocessed to derive the focusing error signal F and tracking errorsignal T. Then the tracking off-set can be reduced by n times and theaccurate tracking control can be performed. Further the focusing errordetecting photodiode and tracking error detecting photodiode arecommonly formed by the same photodiode, so that the number of parts canbe decreased and the cost can be reduced.

When the focusing error detecting photodiode and tracking errordetecting photodiode are commonly formed by the same photodiode, thefocusing error detection and tracking error detection are mutuallyinfluenced and the precision of the error detection might be decreasedto a small extent. In the fourth embodiment these photodiodes are formedby separate photodiodes, so that both the tracking control and focusingcontrol can be performed accurately.

In the fifth embodiment, two beams nearest to the optical axis of theoptical system are selected, and one of these beams is used to detectthe tracking error and the other beam spot is used to detect thefocusing error. The beam spot for detecting the tracking error is madeincident upon the groove formed in the optical disk. The focusing errordetecting photodiode and tracking error detecting photodiode are formedby separate photodiodes arranged integrally with each other, so that themutual influence can be avoided and the tracking control and focusingcontrol can be performed accurately.

Further when the tracking error detection signal according to theinvention and the tracking error detection signal obtained by the knownpush-pull method in which n light beams are made incident upon thetwo-divided photodiode are selectively used, tracking error upon thetrack jump and track access can be reduced.

As explained above, the tracking error detecting apparatus for use inmulti-beam optical disk device according to the invention can remove thedrawbacks of the conventional apparatus in which all of n light beamsreflected by the optical disk are used to detect the tracking error, sothat the tracking error detection is largely affected by the shift ofthe objective lens in the tracking direction with respect to the opticalaxis and the tracking off-set is increased by n times due to theinclination of the surface of the optical disk.

We claim:
 1. A tracking error detecting apparatus for use in amulti-beam optical beam device comprising:an optical means for derivinga plurality of light beams emanating from an optical disk havinggrooves; a first tracking error detecting means including a firstphotoelectric converting element having at least two light receivingregions for receiving a single light beam incident thereto selected fromsaid plurality of light beams derived from said optical means and afirst circuit means for processing output signals from said firstphotoelectric converting element to produce a first tracking errorsignal; a second tracking error detecting means including a secondphotoelectric converting element having two light receiving regions forreceiving all of said plurality of light beams derived by said opticalmeans arranged incident thereto and a second circuit means forprocessing output signals from said second photoelectric convertingelement to produce a second tracking error signal; and a switching meansfor receiving said first and second tracking error signals andselectively supplying said first tracking error signal during recordingand reproducing and said second tracking error signal during a trackjump.
 2. An apparatus according to claim 1, wherein said first trackingerror detecting means further comprising a light beam selecting opticalelement for selecting, among said plurality of light beams derived fromsaid optical beams, a light beam which is nearest to an optical axis ofsaid optical means as said signal light beam.
 3. An apparatus accordingto claim 2, wherein said light beam selecting optical element is a pinhole.
 4. An apparatus according to claim 2, wherein said light beamselecting optical element by a critical angle prism.